Page:Natural History, Fishes.djvu/335

Rh few in number, about fourteen species being the whole of those known to naturalists. They are found both in fresh and salt waters, principally in the northern parts of the world. Six of the number are enumerated as British.

The Lampreys have a smooth, elongated, cylindrical body like that of an Eel. There are seven gill-apertures on each side; the mouth is circular, and its inner surface is studded with hard, crusted tubercles, answering the purpose of teeth. The tongue, which moves backwards and forwards like a piston, has two rows of small teeth. The skin, elevated in a fold around the extremity of the body, answers to dorsal, caudal, and anal fins.

The generic name applied to these fishes, Petromyzon, signifies Stone-sucker; and refers to a curious habit depending on the structure of the mouth. The animal applying its circular lip to the surface of a stone or other solid body in the water, draws in the piston-like tongue; a vacuum is thus produced in the mouth, while the pressure of the super-incumbent body of water causes the lip to adhere to the stone with immense tenacity, until by the protrusion of the tongue the vacuum is voluntarily destroyed.

It is supposed that the Lamprey resorts to this singular expedient to prevent its being constantly carried down by the current of the rivers in which it lives; its powers of locomotion being feeble. But Sir William Jardine has shown that a much more obvious end is effected by the same